This invention relates to a projection device. In particular, the invention relates to a projection device for use in a projection system in which a projected image is formed by spatially modulating light using one or more spatial light modulator devices, and then projecting the spatially modulated light on to a display.
A spatial light modulator is an optical component which is controllable to spatially modulate an incident light beam. One class of spatial light modulators are active matrix devices, which comprise a matrix of individually addressed light modulators each effective to modulate the part of a light beam corresponding to a pixel of the projected image.
Each light modulator may be a liquid crystal, for example as shown in EP 0401912.
Alternatively the active matrix device may comprise an array of the deflectable mirror devices commonly known as deformable mirror devices (DMDs) as, for example, described in U.S. Pat. No. 4,856,863, U.S. Pat. No. 4,615,595 and U.S. Pat. No. 4,596,992. Such deflectable mirror devices comprise an array of miniature mirrored cantilever beam elements which are electrostatically deflectable by electric fields provided across a corresponding array of electrodes. The extent of the deflection can be controlled by means of the electrostatic potential applied to the electrodes to provide variable degrees of deflection. Alternatively the mirror devices can be operated in a binary manner by applying predetermined electrostatic potentials to switch each mirror device between discrete deflection states. Grey scale is then displayed by causing each mirror device to deflect to an orientation in which light is directed either towards a display screen or towards a beam dump for periods corresponding to chosen multiples of a basic period.
Using an array of such mirror devices, each device being individually addressable, a two dimensional image can be produced. The small size and fast switching times of the mirror devices make them usable at video picture data rates, enabling the display of television or video moving images on the display screen.
In a projection system using a deflectable mirror device, the incident light beam does not scan, as in an electron beam in a cathode ray tube, but illuminates the whole array of mirror devices at once. Thus a projection system including one or more deformable mirror devices suffers the disadvantage that it is necessary to space the light source and the projection system from each other such that the light paths of light from the light source to the deflectable mirror or device, and the spatially modulated light transmitted by the deflectable mirror device to the projector system do not cross. This produces limitations on the compactness and efficiency of the overall system.
EP-A-0364043 and EP-A-0418947 both describe a projection system in which an assembly of two prisms between which is an air gap defining a totally internally reflecting surface is interposed in the light paths to and from a reflective liquid crystal spatial light modulator. The totally internally reflective surface is effective to transmit one of the light beams either passing to or from the spatial light modulator, and to reflect the other of the light beams thus separating the two beams.
Our copending International patent application WO 95/22868 (the contents of which are incorporated herein by reference) discloses a projection system including a prism assembly including at least one air gap defining a totally internally reflective surface effective both to deflect incident light onto a deflectable mirror array and to transmit spatially modulated light produced by the deflectable mirror array towards a projection lens system. Such an arrangement enables the light beams incident on and reflected off the spatial light modulator to overlap, thus enabling closer spacing of the light source and projector lens than would otherwise be possible.
According to a first aspect of the present invention there is provided a prism assembly for use in a projection system, the prism assembly including at least one totally internally reflective surface effective to transmit incident light onto at least one reflective spatial light modulator, and to reflect spatially modulated light produced by the spatial light modulator towards a display arrangement.
According to a second aspect of the present invention there is provided a projection device comprising at least one dichroic surface effective to split incident light into different colour component beams, and a plurality of reflective spatial light modulators, wherein the means for splitting the incident light into different colour component beams is incorporated in a prism assembly, said means for splitting light being arranged at an angle relative to the incident light dependent of the spread of angles of light within the incident light.
The spatial light modulators may be deflectable mirror devices. Alternatively the spatial light modulators may be reflective liquid crystal devices.
In one particular embodiment in accordance with the invention, there are provided an equal number of totally internally reflective surfaces to the number of spatial light modulators.
Alternatively a single totally internally reflective surface may be used to deflect light onto and deflect light from a plurality of spatial light modulators.
The totally internally reflective surface may be constituted by an air gap in the prism assembly.